A new study shows that when heavy Internet users go offline, they undergo withdrawal symptoms similar to those experienced by drug users.

The study, carried out by researchers at Swansea and Milan Universities, is the first into the immediate negative psychological impacts of Internet use.

Sixty volunteers, with an average age of 25, were first tested to determine their level of Internet use — particularly, whether they used the Internet obsessively or to the detriment of their social relationships and jobs.

They were then were told to surf the net, visiting any sites they liked, for 15 minutes, after which they were tested for mood and levels of anxiety. The volunteers whom the earlier round of testing indicated were “addicted” to the Internet reported increased negative moods after they stopped surfing the net, suffering a “comedown” that researchers said was not unlike that experienced by people after using the drug ecstasy.

In a press release, Professor Phil Reed, who was involved in the study and is based at Swansea University’s College of Human and Health Sciences, wrote:

“Although we do not know exactly what Internet addiction is, our results show that around half of the young people we studied spend so much time on the net that it has negative consequences for the rest of their lives.”

“These initial results, and related studies of brain function, suggest that there are some nasty surprises lurking on the net for people’s wellbeing.”

A note of caution here: although there are rehab clinics that purport to deal specifically with the problem of Internet addiction, it’s not currently recognized as a psychiatric disorder. “What the American Psychiatric Association have done is flag it up as a potential problem that requires further investigation. That’s the first step in it becoming a true disorder in its own right,” Professor Reed tells TIME. However, he cautions, any of the study’s results relating to “internet addiction” could also be symptoms of other potentially addictive online activities: if someone is addicted to gambling, for example, then they might be using the Internet excessively to access gambling sites. But while Reed acknowledges that Internet addiction may be secondary, “my own view is we’re probably looking at a new disorder here.”